To produce electric energy using wind turbines, rotary electric machines are used to convert kinetic energy to electric energy. In this field, segmented rotary electric machines are also employed (i.e., in which the facing active parts moving with respect to one another are defined by axial active segments, which slide axially into and out of respective seats on respective supports of the rotary electric machine). A segmented rotary electric machine is described in Applicant's U.S. Pat. No. 7,936,102 B1 and U.S. Pat. No. 7,808,149 B2 and U.S. Published Patent Application No. 2010/0123318 A1. This rotary electric machine configuration permits relatively easy assembly of the active parts and relatively easy replacement of the active segments when servicing the machine.
In a wind turbine, the rotary electric machine serves to convert kinetic energy to electric energy. Part of the kinetic energy converted by the rotary electric machine, however, is converted to heat, which must be removed to optimize efficiency of the machine. It should thus be appreciated that as the temperature increases, the efficiency of the rotary electric machine decreases.
The electric energy produced is subsequently transformed in phase and frequency in static electric machines, which are also cooled to optimize performance.
For this purpose, the wind turbines described in U.S. Pat. No. 7,057,305, U.S. Pat. No. 7,161,260, U.S. Pat. No. 6,676,122, U.S. Pat. No. 7,594,800 and EP Patent No. 2,136,077 comprise air cooling systems. More specifically, EP Patent No. 2,136,077 describes a wind turbine comprising a rotary electric machine; a rotary assembly with a hub; blades fitted to the hub; a nacelle supporting the rotary electric machine; and a forced-air cooling system, which feeds air successively through the hub, the rotary electric machine and the nacelle. In other words, air flows in through a first opening in the hub, and out through a second opening in the rear of the nacelle.
Air cooling systems provide for fairly good performance of rotary electric machines on wind turbines installed in relatively mild or cold climates.
In hot climates, on the other hand, liquid cooling systems are required.
U.S. Pat. No. 7,168,251 B1 describes a wind turbine comprising a closed-circuit, liquid cooling system.
Wind turbines must often be designed and built with a cooling system designed according to the climate of where the wind turbine is installed (i.e., to maximize power and efficiency of the electric machine according to the climate at the installation site).
Designing and building wind turbines according to the climate at the installation site, the scale economies made possible by mass production of the wind turbine component parts are greatly reduced.
In this respect, known cooling systems are not particularly versatile and perform relatively poorly as regards to cooling the electric generator.
To eliminate this drawback, Applicant's EP Patent Application No. 2,354,542 proposes a rotor liquid cooling system with a tubular structure fitted with heat exchangers.
This system is relatively highly effective and versatile, but the heat exchangers increase the weight of the rotor and are located on the opposite side to the active segments.
The active segments as disclosed in documents EP Patent No. 2,378,631 and EP Patent No. 2,109,208 normally comprise a laminated pack; and an active member normally defined by a coil or a block of permanent magnets and housed inside a seat on the laminated pack.
One drawback observed by the Applicant is the failure of certain known cooling systems to maintain an even temperature along the active segment. That, the temperature along the active segment varies widely between a maximum and minimum value.